1. Field of the Invention
This invention relates generally to clamping devices, and more particularly, to clamping devices for use with punch and die sets.
2. Brief Description of the Prior Art
The tooling for punching holes into sheets for webs has traditionally consisted of a hardened punch which penetrated into a die fabricated from a softer material. The die had an opening which was considerably larger than the punch but which had been peened to be slightly smaller than the punch. The peened upper surface of the die was then ground flat leaving a thin ridge of material protruding into the die opening beyond the interior walls of the die. This ridge was broached to be the same size as the punch and exactly aligned with the punch by inserting the punch into the die after the two were pinned and bolted into the upper and lower punch jaws of the assembled punch press. This insured a tight fit between a particular punch and a die as mounted on a specific assembled punch press and overcame the difficult task of machining a die to match a punch. Further, the expensive machining problem of establishing a pin position tolerance for the punch on one punch jaw, the die on the other punch jaw, and the two jaws relative to each other through linear motion guideways such that the punch would not interfere with the die and would, in fact, be centered in the die was obviated.
The major drawback to fabricating dies of softer material was that the die did not last long before it needed to be resharpened. In addition, it was discovered that a larger clearance between the punch and die was desirable to improve cut quality on certain types of products so long as the clearance was uniform on all sides of the punch. Machine tool improvements allowed hard die materials to be machined to precisely match a punch contour with a prescribed amount of clearance. However, since the die was now made of a harder material, the method of broaching the die to precisely center the punch in the die was no longer possible. Instead, the die or the punch was rigidly mounted to one of the jaws of the assembled press while optical or other means were used to float the punch into centered alignment with the die. The punch, being in contact with but not yet fixed to the second punch jaw would be free to move in three degrees of freedom, two translational and one rotational. When the punch was centered, the screws for securing it to the punch jaw would be progressively tightened with great care to avoid shifting the punch on the jaw which would result in loss of the desired punch position. This was a delicate operation which often had to be repeated several times as numerous factors could cause the punch to shift two or three microns out of position. The friction between the threads and the punch tend to induce a rotation of the punch during tightening. Further, if the load bearing area of the head of the bolt is not flat and perpendicular to the axis of the bolt, or if the head seating area on the jaw is not flat and parallel to the head of the bolt, then a large lateral force can exist upon tightening which will cause the punch to translate slightly. Even the effort of applying a force to a wrench to tighten the bolts can cause some lateral displacement of the punch.
Cam actuated devices are well known for their advantages of quickly and securely clamping materials together in a single motion and are often used as compliments to or substitutes for screw-type fasteners. Prior art examples of cam actuated clamping devices for attaching one member to others are disclosed by U.S. Pat. No. 5,156,508 to Grisley and U.S. Pat. No. 5,257,855 to Nagano. These devices include a course adjustment means using a threaded draw bar to bring the parts to be clamped into sufficiently close abutment such that the limited additional displacement afforded by the cam brings the parts into a tightly clamped relationship. Both the Grisley and Nagano devices produce the cam actuated clamping motion by rotating a cam lobe having a cam surface which is increasingly eccentrically offset from the pivot point of the cam. However, upon actuating the cam, the sliding action of the cam surface against the abutting plate gives rise to a friction force having a magnitude on the order of 60 percent of the clamping force. This force, acting tangentially at the point of contact between the cam surface and the abutting plate is generally perpendicular to the axis of the tie rod thereby generating opposing laterally directed forces on the abutting plate and the tie bar. U.S. Pat. No. 4,010,669 to Kloren teaches a bolt tensioning arrangement comprising a plurality of ring-shaped elements having interacting conical surfaces. One of such ring-shaped elements is radially collapsible. A plurality of circumferentially spaced screws are uniformly tightened stepwise in circumferential order to axially draw the ring-shaped elements together which, in turn, put the bolt in axial tension. The result is a time consuming procedure requiring precision tightening of multiple screws in a prescribed sequence for each clamping bolt.